1. Field of the Invention
The present invention relates to a photoresist resin useful for forming fine patterns in very large scale integrated circuit (hereinafter referred to as "VLSI"). More particularly, the present invention relates to an acetal group-containing alkoxy-styrene polymer useful for chemical amplified photoresist of high sensitivity and resolution. Also, the present invention is concerned with a method for preparing the polymer and a chemical amplified photoresist composition mainly comprising the same.
2. Description of the Prior Art
Recently, there has been a need for fineness of pattern rule due to high integration and high speediness of large scale integrated (LSI) circuit. However, there is a limitation for forming fine patterns, because light has long wavelength, which irradiated from light sources as used in usual exposure techniques.
For this reason, more recently, new light sources, for example, g line (wavelength 436 nm) or i line (wavelength 365 nm) irradiated from ultra-high pressure mercury lamp have been developed. In this case, however, the pattern rule is, limited to a resolution of about 0.5 .mu.m. Thus, LSI having an integration degree of at most 16 MDRAM could be prepared.
Still more recently, intensive research has been made to overcome such limit as to the integration degree. As a result, deep-UV having shorter wavelength than that of g line or i line was found to be useful as a light source and deep-UV lithography using the light source was promising. The deep-UV lithography makes it possible to form pattern rule with a resolution of 0.1 to 0.3 .mu.m. Meanwhile, LSI can be produced in a large scale by a lithographic technique using KrF excimer laser as a light source. Thus, resist materials corresponding this technique are required properties of low photo-absorption and high sensitivity as conditions for resist materials.
Meanwhile, chemical amplified resists were developed as resist materials having high resolution and high resistance to dry etching as well as having sensitivity equivalent or higher than that of conventional high sensitivity resist materials. For example, a three-component chemical amplified negative resist material consisting of a novolak resin, a melamine compound and a photo acid-generating agent was already commercially produced by Sipley, sold under the tradename of "SAL601 ER7". Another example of chemical enhanced resist material, developed by CIBA Geigy, comprises an acetal protected by aromatic ring aldehyde, an onium salt and a phenolic resin, as disclosed in SPIE vol. 1925, p.109. The protected acetal serves as a crosslinking group for the phenolic resin. It is reported that this resist material is not only highly transparent and sensitive to deep UV, but can be subjected to alkali development.